1. Technical Field
The present invention relates generally to a system and process for converting a hydrocarbon gas to a hydrogen-containing gas and, more particularly, to a system and process for combusting a hydrocarbon gas with air to produce a reformed gas containing hydrogen and carbon monoxide.
2. Background Information
A need has long existed for converting available carbonaceous materials to intermediates that can subsequently be converted to scarce, but useful, hydrocarbon products such as liquid hydrocarbon fuels, petrochemicals and the like. For example, coal is one such carbonaceous material that is readily available in some locales. U.S. Pat. No. 3,986,349 teaches a process for gasifying coal to an intermediate synthesis gas that can subsequently be hydrogenated to provide a valuable liquid hydrocarbon fuel. The fuel is used to generate power by relatively clean combustion in an open-cycle gas turbine.
Natural gas is another carbonaceous material that is plentiful in many regions, yet uneconomical to develop because of the lack of local markets for the gas or the high cost of transporting the gas to alternate markets. One solution is to produce the natural gas and convert it in the field to a more utilitarian liquid hydrocarbon fuel or other liquid product. The conversion product can be used locally or cost-effectively transported to alternate markets. Processes for converting light hydrocarbon gases, such as natural gas, to heavier hydrocarbon liquids are generally known in the prior art. Such processes typically involve the "indirect" conversion of methane to synthetic paraffinic hydrocarbon compounds, wherein methane is first converted to an intermediate synthesis gas containing hydrogen and carbon monoxide. The resulting synthesis gas is then converted to liquid synthetic paraffinic hydrocarbon compounds via a Fischer-Tropsch reaction. Unconverted synthesis gas remaining in the process tail gas after the Fischer-Tropsch reaction is usually catalytically reconverted to methane via a methanation reaction and recycled to the process inlet to increase the overall conversion efficiency of the process.
Conversion of methane to a synthesis gas is often performed by high-temperature steam reforming, wherein methane and steam are reacted endothermically over a catalyst contained within a plurality of externally-heated tubes mounted in a large fired furnace. Alternatively, methane is converted to a synthesis gas via partial oxidation, wherein the methane is exothermically reacted with purified oxygen. Partial oxidation using purified oxygen requires an oxygen separation plant having substantial compression capacity and correspondingly having substantial power requirements. Production of the synthesis gas via either of the above-recited means accounts for a major portion of the total capital cost of a plant converting methane to paraffinic hydrocarbons.
Autothermal reforming is a lower cost means of converting methane to a synthesis gas. Autothermal reforming employs a combination of partial oxidation and steam reforming. The heat required to activate the endothermic steam reforming reaction is obtained from the exothermic partial oxidation reaction. Unlike the above-recited partial oxidation reaction, however, air is used as the source of oxygen for the partial oxidation reaction. In addition, the synthesis gas produced by autothermal reforming contains substantial quantities of nitrogen from the inlet air. Consequently, it is not possible to recycle the unconverted components contained in the process tail gas without undesirably accumulating an excess of nitrogen within the process. Production of a nitrogen-diluted synthesis gas via autothermal reforming or partial oxidation using air followed by conversion of the synthesis gas via a Fischer-Tropsch reaction as disclosed in U.S. Pat. Nos. 2,552,308 and 2,686,195 is, nevertheless, a useful means for obtaining synthetic hydrocarbon liquid products from methane.
U.S. Pat. No. 4,833,170 discloses another example of autothermal reforming, wherein a gaseous light hydrocarbon is reacted with air in the presence of recycled carbon dioxide and steam to produce a synthesis gas. The synthesis gas is reacted in the presence of a hydrocarbon synthesis catalyst containing cobalt to form a residue gas stream and a liquid stream comprising heavier hydrocarbons and water. The heavier hydrocarbons are separated from the water and recovered as product. The residue gas is catalytically combusted with additional air to form carbon dioxide and nitrogen which are separated. At least a portion of the carbon dioxide is recycled to the autothermal reforming step.
Prior art hydrocarbon gas conversion processes may be adequate for converting hydrocarbon gases to reformed gases, such as synthesis gas, having utility as intermediates in the production of desirable end products. Nevertheless, such processes have not been found to be entirely cost effective due to significant capital equipment and energy costs attributable to compression of the inlet air. The power required to compress the inlet air represents the majority of the mechanical power required to operate the process, yet much of this power is essentially lost as unrecovered pressure energy in the intermediate reformed gas or off-gas from the process. In addition, significant chemical fuel energy in the form of unconverted compounds and unrecovered products is frequently retained in downstream residue gases. The generally highly dilute nature and low heating value of downstream residue gas inhibits efficient recovery of the fuel energy therefrom. As a result the fuel energy is oftentimes discarded or recovered only with extreme difficulty and expense.
Another drawback experienced with prior art hydrocarbon gas conversion processes, and in particular with autothermal reforming or partial oxidation, is the limited yields of desirable intermediates resulting therefrom. Although the autothermal reforming or partial oxidation reactions approach equilibrium at high temperatures, a significant degree of reverse reaction occurs during the subsequent cooling/quenching step diminishing the net yield of intermediates. Thus, it is apparent that a need exists for a more effective hydrocarbon gas conversion process overcoming the above-described drawbacks of prior art processes.
Accordingly, it is an object of the present invention to provide an effective process for converting a hydrocarbon gas to a reformed hydrogen-containing gas. It is also an object of the present invention to provide an effective system of process equipment for converting a hydrocarbon gas to a reformed hydrogen-containing gas. More particularly, it is an object of the present invention to provide such a hydrocarbon gas conversion system and process having substantially reduced power requirements. It is another object of the present invention to provide such a hydrocarbon gas conversion system and process having substantially reduced capital equipment costs. It is yet another object of the present invention to provide such a hydrocarbon gas conversion system and process effectively utilizing the pressure energy of an off-gas and/or the fuel energy of a downstream residue gas. It is a further object of the present invention to provide such a hydrocarbon gas conversion system and process having improved yields of desirable products. These objects and others are achieved in accordance with the invention described hereafter.